A method of reducing wastage during the start-up phase of an extrusion device used to produce extruded profiles such as tread strips for vehicle tires from a plurality of mixtures is disclosed. The extrusion device comprises a plurality of extruders each extruding a component of the profile and each extruder has its own drive means. The outlets from the extruders communicate with a template member having flow channels formed therein which lead into a common extrusion port. The drive means are actuated simultaneously and the pressure of each of the components is monitored in the outlet region of each extruder. These pressures are compared with a pre-determined desired pressure by a control device, which device switches-off the appropriate drive means as each desired pressure is attained. When all desired pressures have been attained, all of the drive means are re-actuated simultaneously by the control device. In a minor modification of the method, the drive means for the final extruder to reach its desired pressure is not switched-off and the other drive means are simultaneously re-actuated when the final desired pressure is attained.
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A coextrusion system employs two or more extruders, each with a gear pump at its output connected to a coextrusion die. Different materials are extruded by each extruder. Varying the speed of one or both gear pumps varies the content of the extrudate. The gear pumps permit precise variation of the relative content of the materials extruded by the several extruders lengthwise along the extrudate. A display of a cross sectional dimension of the extrudate along a length of the extrudate permits observation of the lag that occurs between alteration of gear pump speed (or air pressure in the case of tubular or blown sheet extrudates) and the resultant change in dimension, so that a speed correction can be made at the appropriate time to compensate for differing shrinkage and stretching characteristics between the several extruded materials.
A coextrusion system employs two or more extruders, each with a gear pump at its output connected to a coextrusion die. Different materials are extruded by each extruder. Varying the speed of one or both gear pumps varies the content of the extrudate. The gear pumps permit precise variation of the relative content of the materials extruded by the several extruders lengthwise along the extrudate. A display of a cross sectional dimension of the extrudate along a length of the extrudate permits observation of the lag that occurs between alteration of gear pump speed (or air pressure in the case of tubular or blown sheet extrudates) and the resultant change in dimension, so that a speed correction can be made at the appropriate time to compensate for differing shrinkage and stretching characteristics between the several extruded materials.
An extrusion apparatus and method for producing a composite profile strip characterized by an extrusion head having plural flow channels extending to an extrusion die and at least two extruders for feeding respective components at respective different pressures into the flow channels for passage to the extrusion die. The extrusion die has a common die outlet from which the components are discharged to form the composite profile strip, and respective flow restriction devices are disposed in the flow channels between the extruders and the extrusion die. The flow restriction devices are operative to decrease the difference in pressure between the components before being brought together at the extrusion the extrusion die and further to dampen the effect changes in extruder speed has on the profile of the composite profile strip.
